1. Field of the Invention
The present invention relates generally to device interconnection techniques, and more specifically to an apparatus for attachment of semiconductor or superconducting chips directly to circuit boards or multi-chip modules.
2. Description of the Prior Art
Conventional techniques for interconnecting semiconductor chips to substrates include ball-grid array (BGA), micro-ball grid array (.mu.BGA) and "flip-chip" bonding. These techniques traditionally use a conductive pattern formed on a surface of a semiconductor chip that is mounted and mated to a corresponding conductive pattern formed on a surface of a substrate. Typically, the conductive patterns include bump-like mating pads formed of solder, gold or epoxy. To perform the chip-to-substrate attachment a bonder instrument may be used. Conventional bonders use various techniques to achieve parallel alignment between the chip and the substrate. Parallel alignment is required to ensure that the bump-like mating pads formed on the chip, mate with corresponding pads formed on the substrate where each chip bump makes full surface area contact with equal amounts of pressure to each corresponding substrate bump. The result of improper parallel alignment between the chip and the substrate is unconnected mating pads and hence open circuits.
Present bonders may employ either of two techniques for making a chip parallel with a substrate or board. A first technique involves using cameras and targets. A chip and a substrate 12 are separated by approximately a one-half inch gap and an assembly of prisms and camera lenses 14 is inserted into the gap. Through the assembly 14, an indicia, such as a cross, is projected onto the chip 10 and the substrate 12 respectively. The cross is reflected back from the surfaces of the chip 10 and substrate 12 onto a camera lens 14. As illustrated in FIG. 1a, the bonder system is calibrated so that if the chip 10 is parallel with the substrate 12, the cross image 18 reflected back from the chip surface will overlie with the cross image 20 reflected back from the substrate surface. Conversely, as illustrated in FIG. 1b, if the chip 10 is not parallel with the substrate 12, the reflected cross images (18, 20) will not overlie, and a series of mechanical or hydraulic controls may be used to bring the two crosses to the point where they align. Following the parallel alignment of the chip and the substrate, the prism and lens assembly 14 is retracted and the chip 10 is brought down to the substrate 12 for attachment.
The system previously described has several disadvantages, including limited angular precision and incompatibility with smaller contact pads, and lack of functionality when used with non-reflective chip or semiconductor surfaces. Specifically, conductive pads used in industry are from approximately 100 .mu.m to 150 .mu.m in height. To increase the data transmission rate through a conductive pad or bump interconnection, the height of the conventional bump must be decreased from the 100 .mu.m to 150 .mu.m range down to as small as 2 .mu.m. Shorter pads require a tighter angular precision to orient the chip parallel with the substrate, and conventional camera and lens bonding systems have limited angular precision. Conventional camera and lens systems also require reflective chip and substrate surfaces so that the cross can be reflected back from the chip and substrate to the camera. Many substrate materials do not reflect light clearly or have multi-faceted surfaces that reflect light in multiple directions with equal or varying intensities. The effect is difficulty or impossibility in choosing the correct crosses to align.
A second technique uses a floating chuck assembly. Without a chip or a substrate in the system, a chuck is brought into contact with a flat plate on which the substrate will be disposed. Pressure is applied until the two flat surfaces are parallel, the chuck is locked into position either using a vacuum or a mechanical means, and the chip and substrate are placed into the assembly. This technique is successful only to the extent that both the substrate and the chip have front to back side parallellity. Additionally, this technique would not be suitable for use with the substrate and chip already in place, since the pressure used to assure the appropriate degree of parallellity, combined with the weight of the chuck assembly, would likely damage fragile chip components. As mentioned previously, this technique may employ locking the chuck into position by using a vacuum. A vacuum nipple and hose are typically connected directly to the floating portion of the chuck and this configuration applies unwanted torque on the floating portion of the chuck, thereby resulting in increased angular precision errors.
Based on the techniques known in the art for bonding chips to printed circuit boards or multi-chip modules, a bonding apparatus that increases the precision of the chip to board attachment while reducing the complexity of overall bonding process is highly desirable.